Since the discovery of the first cyclopentadienyl transition metal compound, ferrocene, considerable effort has been expended to evaluate the effects of substituents located on the cyclopentadienyl moiety (Cp) on the electronic character of the transition metals complexed thereto. In terms of substituents, most attention has been devoted to the pentamethylcyclopentadienyl moiety (Cp*).
Pentamethylcyclopentadiene was first synthesized in 1967 by R. B. King et al., J. Organomet. Chem., 8, 287 (1967). Since, it has been widely used as a ligand for a broad range of transition metals. See, for example, P. M. Maitlis et al., Acc. Chem. Res., 11, 301 (1978). The presence of the methyl groups serves two purposes. First, the methyl groups sterically encumber and hinder the complexed transition metal, thus protecting it from attack by bulky reagents. Second, the five methyl groups significantly alter the electronic character of the complexed transition metal through electron donation by the methyl groups which is enhanced relative to that provided by hydrogen as a substituent on cyclopentadiene complexes. See P. G. Gassman et al., Organometallics, 2, 1470 (1983).
By comparison, relatively little is known about the properties of transition metal complexes incorporating cyclopentadienyl ligands bearing electron-withdrawing substituents. See D. W. Macomber et al., Advances in Organometallic Chemistry, 21 (1982) at pages 4-28.
The preparation of a few transition metal complexes comprising trifluoromethylcyclopentadienyl ligands has been reported. For example, the synthesis of trifluoromethylcyclopentadienyl(cyclooctadienyl) cobalt from a halotris(triorganophosphine)cobalt(I) complex has been reported, and trifluoromethylcyclopentadienyl thallium has been patented (U.S. Pat. No. 4,699,987). In 1977, A. Bond et al., cited below, reported the preparation of the dimer of 1-perfluoroalkyl-2,3,4,5-tetramethylcyclopentadienyl-iron tricarbonyl via the irradiation of a solution of tetramethylcyclobutadientyliron tricarbonyl and trifluoromethylacetylene, followed by thermolysis of the reaction product. Tetrakis(trifluoromethyl)cyclopentadienide complexes of ruthenium have also been prepared by M. J. Burk et al., J. Amer. Chem. Soc., 111, 8939 (1989). However, cyclopentadienyl ligands bearing trifluoromethyl groups have not been widely developed as transition metal ligands. This is surprising, in view of the desirability of the use of cyclopentadienyl complexes which include electron-withdrawing substituents in certain catalytic processes. For example, the ability of a series of (cyclopentadienyl)(cyclooctadienyl)cobalt complexes to catalyze pyridine and/or xylene ring formation from a mixture of ethyl cyanide and propyne was found to increase as the electron-withdrawing strength of the substituent on the cyclopentadienyl ring increased. Increased electron density at the cobalt atom resulted in a reduction of the catalyst activity of the cobalt complex. In contrast, electron-withdrawing substituents lowered the electron density at the cobalt atom, and the deshielded cobalt "core" exhibited higher catalytic activity. Thus, in this series, the pentamethylcyclopentadienyl system exhibited the lowest catalytic activity in the test reaction, whereas the highest activity at 65% propyne conversion was found for the benzoylcyclopentadienyl system, which was 1,000 times more reactive. See H. Bonnemann, Angew. Chem. Int. Ed. Engl., 24, 248 (1985).
Because electronic effects can dramatically influence chemical behavior, a need exists for cyclopentadienide ligands with the steric bulk provided by pentaalkyl (for instance, pentamethyl or pentaethyl), in combination with the relative electronic character of unsubstituted cyclopentadienide or the electron deficiencies provided by mono- or di-trifluoroalkyl substituents. Transition metal complexes wherein the transition metal is complexed to such sterically encumbered but electronically neutral (or electron deficient) ligands may be useful in a wide variety of catalytic processes, such as the Ziegler-Natta polymerization of ethylene and propylene, acetylene trimerization, carbon-hydrogen activation, hydrogenation and the like.